Conventionally, to extend duration term of the metal parts by improving abrasion resistance and/or corrosion resistance of various metal parts such as casting molds and rolls used in a steel manufacturing process, wheels for automobiles, and components for gas turbines, it is popular to form a cover layer composed of nickel, copper, aluminum, chrome, an alloy of these metals, or the like.
As a method for forming a cover layer, a metal plating method is applicable. However, hardness in forming of the cover layer for a large area and crack generation in the cover layer might arise as a drawback of the metal plating method.
As another method, a thermal spray deposition method can be exemplified in which cover layer is formed by thermal splay deposition. In the thermal spray deposition method, low pressure plasma spray (LPPS) deposition method, a flame spray deposition method, a high velocity flame spray (HVOF) deposition method, and an atmospheric plasma spray deposition method are included. However, when a cover layer is formed by these methods, metal is oxidized during spraying. As a result, low electric conductivity and low thermal conductivity caused by difficulty in forming of a dense cover layer, lower economical profit caused by low deposition efficiency and the like have been pointed out as a drawback.
Recently, “cold spray system” in which a cover layer is formed by using raw material powder in a solid-phase state has been paid attention to as a new technology for forming a cover layer in place of the methods described above. In the cold spray system, a working gas having temperature lower than a melting point or a softening point of the raw material powder is made to be a supersonic flow, and a raw material powder carried by a powder feed gas is injected into the working gas from a tip of a powder port to make the raw material powder strike against a substrate in the solid-phase to form a cover layer. In other words, the cold spray system is a method to strike raw material powder of a metal, an alloy, an intermetallic compound, or a ceramics against a substrate surface at high speed in the solid-phase state to form a cover layer. A cover layer forming method employing the cold spray system is hereinafter referred to as “CS method” to distinguish the cover layer forming method from the plasma spray deposition method and the like described above.
A concept of the CS method will be demonstrated in detail with reference to FIG. 2 as a schematic diagram of a typical cold spray system and FIG. 3 as a schematic sectional view showing an example of a conventional nozzle for cold spray system. Gas supply line connected to a compressed gas cylinder 2 in which nitrogen gas, helium gas, air, and the like are stored is branched into a working gas line (the line through a valve 5a) and a powder feed gas line (the line through a valve 5b). High-pressure working gas to be introduced into a chamber 12 of a cold spray gun is elevated a temperature equal to or lower than a melting point or a softening point of raw material powder by the heater unit 10. On the other hand, high-pressure powder feed gas is introduced into the raw material powder feeder 15 to carry the raw material powder into the chamber 12. The raw material powder carried by the powder feed gas is supplied from the tip of the powder port 1h and is made to be a supersonic flow by the working gas while passing a conical convergent shape part 1b to a throat part 1c and then the raw material powder is shot from a spout 1e provided at the tip of a conical divergent shape part 1d to strike against the surface of a substrate 18 while keeping the solid-phase state and then a cover layer is formed.
It is well known that the cover layer formed by using the CS method comprises fine grains in high density, high electric conductivity and high thermal conductivity, less oxidation and less thermal modification and excellent adhesion with the substrate in comparison with the cover layer formed on the substrate by using the thermal spray deposition methods described above.
An object to be solved in the CS method is that all of a raw material powder shot from the nozzle tip cannot be consumed to form a cover layer on the substrate surface. In other words, efficiency of formation of a cover layer by a shot raw material [(amount of raw material powder consumed to form a cover layer)/(amount of shot raw material powder)]×100% (hereinafter referred to as “spray efficiency”) cannot reach to 100%. In addition, when the spray efficiency is small, the raw material powder not consumed to form the cover layer scatters around the substrate, i.e. it may results a waste of resources and energy. Further, longer operation time may be required for a cold spray device for forming an objective cover layer. It means that if the spray efficiency is increased, cover layer formation efficiency is improved and the raw material powder that scatters after missing formation of a cover layer might be reduced. In other words, productivity of the cold spray device is improved and, at the same time, resources and energy can be effectively utilized.
Therefore, Patent Document 1 discloses a technology considering that higher temperature of the raw material powder is preferable as long as the temperature is lower than the melting point, the raw material powder and the working gas just before the raw material powder strikes against the substrate are heated up to elevate the temperature of the raw material powder and, at the same time, to increase a linear velocity of the gas. Specifically, the raw material powder is induction-heated by using a microwave in the region between the vicinity of the tip of the divergent shape part and the substrate surface. The effect of the heating disclosed is an increased deformation of the powder on the substrate surface. In such a way, when the deformation of the powder on the substrate surface is made big, the spray efficiency of the CS method may be increased.    [Patent Document 1] US Patent Publication 2006-27687